An electrophotographic type image-forming device applies a transfer bias to a transfer unit to transfer an image formed on an image-bearing member onto a paper nipped between the image-bearing member and the transfer unit. In order to form a clear image with such an image-forming device, it is important to apply an appropriate transfer bias to the transfer unit. If the transfer bias is less than the appropriate transfer bias, the force of attraction or adherence of toner to paper is insufficient. This may result in scattered toner and ghost images produced by residual toner on the image-bearing member. Conversely, if the transfer bias is excessive, an electric discharge can occur between the image-bearing member and the paper. The electric discharge can damage the image-bearing member or produce a discharge pattern in the transferred image.
An appropriate transfer bias is determined based on the electrical resistance of the transfer system including the transfer unit, the image-bearing member, and the paper. On the other hand, these resistances change in accordance with variations in ambient conditions, and particularly in temperature and humidity. Therefore, the appropriate transfer bias also changes in accordance with variations in ambient conditions. The invention disclosed in Japanese unexamined patent application publication No. 2006-53175 detects the resistance of the transfer system, determines an optimal transfer current for the detected resistance by referring to a predetermined characteristic curve indicating an optimal transfer current for each resistance, and matches the transfer current flowing in the transfer roller to the determined optimal transfer current.